If it had only been from Marinette and her perspective only, she wouldn't have chosen Félix personally. But she didn't chose him. It is more due to how the events turn so.
In Emotion, Ladybug heard why he did what he did. That a lot of things he has done are related to secure that one specific Miraculous.
Félix also seems to have learn in lesson about using the Peacock Power out of anger against humanity. Ladybug didn't need any Lucky Charm nor specific Miraculous to defeat him during Emotion. Only words. And Adrien AND Kagami were the ones who were able to reason with him and force him to return everyone back.
In Pretension, Kagami herself took his defence despite her first reluctance.
In both Emotion and Pretension, Ladybug was more than ready to recover the Peacock. But she put her personal feelings aside and trusted Kagami's judgment to let go Félix... hoping she wouldn't regret it. And Kagami also asked Félix to trust Ladybug in return before he escaped.
And, in the end, Félix did side with Ladybug and not Monarch by telling about the truth about Gabriel so he could be defeated, even if Félix had other motivation (be with Kagami).
So, for now, yes, it feels more like some sort of truce between parties. Marinette now knows that Félix is a sentibeing, so he is one of the rare people who can understand what the Peacock power involves on a very personal level. And she seems to know that - if he can help it - Félix won't create a life on a whim, just so to kill it afterwards. And she also knows that Félix will also protect that Miraculous with his life, because his life would be at stake if someone badly intention would take the Peacock Miraculous from him only to wipe him out his existence.
So this partly explains it. But I would also guess there is another thing at play here and it would be hope. The hope that Félix would become a better person because she is accepting to give him a second chance and prove himself. Just like she did with Chloé or Sabrina. Both Chloé and Sabrina received a Miraculous despite not having done anything particularly remarkable to deserve it. And yet, Ladybug gave them each a chance. One was a huge deception and the other is living it up to that hope.
Besides, Marinette is still the Guardian. She will recover the Miraculous if Félix is acting up and misusing the privilege. But right now, he seems to be treating Duusu well and staying put.